This invention relates to the field of chalk line assemblies wherein a length of line, enveloped in chalk dust, can be withdrawn from a housing and extended to mark a straight line. The line is then automatically drawn back into the housing without recourse to hand cranking. The typical mechanism involves a coiled spring within the housing which causes the retracting action. Various types of locking mechanisms are used to hold the line in extended position. Many such devices are known in the art.
A well known problem encountered with automatically retracting line assemblies is that the speed of retraction, due to the requirement of having a sufficient amount of stored energy in the coiled spring, is usually greater than desired by the operator for safe retraction of the line. Retracting the line results in a high velocity projectile, the line end piece, snapping back at the operator. This can result in injury to the operator or the device itself.
A review of prior art indicates that various mechanisms have been employed to either lock the line in extended position or to slow the retraction speed. Some mechanisms accomplish both purposes. In this application, for ease of understanding, the terms "lock" or "locking" will be used for mechanisms that hold the extended line in a stationary position. The terms "brake" or "braking" will be used for mechanisms that slow the retraction of the line. In the prior art, the terms are sometimes used interchangeably, so examination of the function is necessary.
Representative locking mechanisms are taught by Gavin in U.S. Pat. No. 627,057, Field in U.S. Pat. No. 845,401, Kiso in U.S. Pat. No. 997,831, Kane in U.S. Pat. No. 3,114,515, Quenot in U.S. Pat. No. 4,121,785, and Karger in U.S. Pat. No. 4,565,011. Gavin teaches a gear tooth lock that mechanically engages teeth on the spool. Field teaches a brake-shoe lever which locks the reel. Kiso uses a ratchet and pawl mechanism which locks the line in extended position. Kane uses an internal leaf spring which frictionally engages the side of the drum and contact is broken by pushing on a central button. Quenot shows a push button lock which causes frictional engagement between the drum and the interior of the button itself. Karger teaches the use of additional internal springs to lock the spool in place. None of the above references teach any mechanism for braking the retraction speed of the line, and the lock mechanisms as taught are not suitable for this purpose.
There are prior patents that teach braking mechanisms or combined braking-locking mechanisms, for example, Chow in U.S. Pat. No. 3,197,155, Petrie in U.S. Pat. No. 2,463,303, Lore et al. in U.S. Pat. No. 4,192,078, and Longenette in U.S. Pat. No. 4,592,148.
Chow shows a device for tightening shoelaces that retracts two lines at once using a coiled spring. The locking part of the device works oppositely from the mechanisms of the chalk line devices, in that the lines are always locked in non-use and the locks are disengaged to both retract and extend the lines. The locks are short rods projecting towards the drum and held in place by springs. Pressing inwardly on the rods releases the lock. Braking is performed by pressing a finger against a protruding pivot shaft, while at the same time pressing both locking rods.
Petrie teaches a reel mechanism that incorporates a friction brake. A disc of material made of leather, rubber, etc. is pushed against side of the spool by a thin metal disc having a central button exposed through the housing. To brake the line during retraction, the user presses the button with his thumb, which in turn presses the material disc against the side of the spool. The greater the pressure, the slower the line is retracted. A locking mechanism consists of a slidable piece which is attached by a rivet to the metal disc, the sliding piece being wedge shaped and, when extended, forcing the metal disc against the material disc. All of these components are contained within the housing, except for the protruding portion of the button.
Lore et al. shows an automatically retracting chalk line assembly that is locked by sliding a tongue against the squared side of the shaft. To brake the retraction, the tongue is only slightly pushed into place, such that the rounded end of said tongue bumps against the rotating corners of the shaft.
Longenette teaches a chalk line assembly where a closed horse-shoe shaped clutch can be pressed against the interior sleeve by action of a slide on the outside of the housing. Progressively sliding the outside slide member varies the amount of braking.
The above described devices all have specific drawbacks which are overcome by this invention. Some of the devices require separate locking and braking mechanisms. They all involve additional parts and constructions, which affect the cost and ease of manufacture, and provide extra sources of mechanical problems for the user. The braking mechanisms all involve frictional contact between parts of the asssembly, so that wear will shorten the effective life of the device in use. Furthermore, any mechanisms which exert pressure against the spool act to deform or misalign the spool with regard to its spindle, causing problems in the retraction efficiency.
This invention provides for an automatically retracting chalk line assembly having a locking/braking mechanism which is an improvement over known prior art. The invention is cheaper and easier to produce, has less parts requiring less assembly, is easier to use in practice and is not subject to wear and mechanical failure.